Method of protecting the surface of articles formed from a magnesium alloy



Patented Mar. 310, 1948 UNIT METHOD OF PROTECTING: SURFACE E:

ARTICLES" FORMED FROM'A MAGNESIUM ALLOY Charles William Clark, Westmount, Quebec, and Warren Arthur Sheaffer, Montreal, Quebec, Canada, assignors to Canadian Copper Refiners Limited, Montreal East, Quebec, Canada, a body corporate No Drawing. Application March 2'7, 1944, Serial No. 528,360

4 Claims.

This invention relates to improvements in the method of protecting the surface of articles formed from a magnesium alloy and the objects of the invention are to provide a simple and eifective method for protecting the surface of the magnesium alloy from corrosion. It consists essentially in forming a protective compound on the surface by immersion in a suitable bath as hereinafter described,

Before the article is immersed in the bath the surface must be prepared by cleaning and degreasing by any of the usual methods. A somewhat different treatment may be employed when the surface is a cast surface. In the preparation of a casting surface oxide scale is removed by immersion in 10% HNOs at 25 C. The immersion is continued until the surfaces are bright. This generally requires from 30 to 60 seconds. The pickled surfaces are rinsed with running Water to remove residual I-INOs. In the preparation of a machined surface the surface is degreased in one of the following baths:

( a) NazCOa-NaOH-soap (b) N a2C03-N asPO4.12H2O (c) KOH The cleaning solutions are kept at 90 centlgrade to boiling. The solution concentration and the time of treatment vary with the initial surface conditions with respect to grease and dirt. Castings which are to be machined should have their oxide scale removed by immersion in 10% nitric acid prior to machining,

The coating bath in which the articl is to be immersed may be prepared from selenious acid and potassium permanganate in the following proportions,

20 gm. S602 10 gm. KMnO4 per litre of water solution. The reagents are dissolved in water separately. The two solutions are then mixed and the volume diluted to the required concentration. The resultant precipitate of an oxide of manganese is filtered oil. The filtrate which comprises the soluble reaction products of the above named ingredients in aqueous solution constitutes the coating solution.

In efiecting the coating, the coating solution prepared as above described is kept at a temperature of from 90 centigrade to boiling. The

cleaned samples of magnesium alloy are immersed in the solution for from 5 to 15 minutes with an average time of 7 minutes. If desired, after removal from the bath, a subsequent immersion in the bath may be given to increase the coating adherence. This adherence may also be increased by one or other of the three following treatments:

(1) Immerse in 15% NazCr2Om2I-I2O at 90 centigrade to boiling for a period of 30 to minutes. Wash and dry.

(2) Coated specimen is dipped in distilled water to remove adhering coating solution. The rinsed metal is heat treated at 150 centigrade for from 1 to 24 hours.

(3) A dichromate boil followed by heat treatment, as in (1) and (2) above.

The process above described, as well as being applicable to magnesium alloys, is applicable to magnesium itself and the term magnesium as used hereinafter in the claims is intended to include alloys of magnesium.

The selenium-containing coating on the magnesium article will be found to be adherent and of a red color and to afford a very substantial protection to corrosion.

We claim:

1. The method of forming a selenium-containing protective coating on an article, which is com.- posed principally at least of magnesium, comprising the steps of cleaning the surfaces of the article to be coated, preparing an immersing solution in the following proportions: dissolving 20 grams of selenium dioxide in water, separately dissolving 10 grams of potassium permanganate in Water, mixing the solutions thus made, diluting the mixed solutions to a total volume of 1 liter, and filtering off any precipitate formed to leave the filtrate as an immersing solution, and immersing the article in a solution as thus prepared while the solution is at a temperature of about C. to its boiling point and for a period of about 5 to 15 minutes.

2. The method in accordance with claim 1 for forming a protective coating, comprising the additional steps of removing the article from the solution, and subsequently re-immersing the article in a solution of the same character for a period such as to give a coating of predetermined increased adherence.

3. The method in accordance with claim 1,

3 7 comprising the additional step of increasing the REFERENCES CITED 7 adherence of the coating by subsequently The following references are of record in th mersing the article in an aqueous solution conme of this patent. 7 taining about 15% sodium bichromate.

4. The method in accordance with claim 1, 5 UNITED STATES PATENTS comprising the additional steps of rinsing the Number Name 7 Date article in distilled water subsequent (20 the ap- 1 9 1 030 Bengough May 29 1934 plication of a coating as aforesaid, and then dry- 2,065,611 Chris/0y Jam 5, 1937 ing it and maturing the coating by maintaining it 2,067,214 Tanner et a1 Jam 12, 1937 at a temperature of about 150 C. for a period of 1 10 2,067,216 Thompson et a1 Jan 12, 7 to 24 hours 2,186,085 Wein Jan. 9, 1940 CHARLES WILLIAM CLARK- 2,303,350 Fuller Dec. 1, 1942 WARREN ARTHUR SHEAFFER- 2,322,203 Loose et a1 June 22, 1943 

